I'm assuming (as much as I hate to do thatT that the 'destination' is not on the included maps available on the iPhone or the car?
I usually just use Google Maps to get started and then drag and drop the route to exactly what I want. Print out the text directions and sometimes a 'street view' of the destination. By the time I get near the end, the map in the car can show me most of what I want, but you can't beat a relatively new 'street view'. If you have friends nearby, I have asked them to stand outside my destination and wave a yellow and a red flag (never use only one colored flag!
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Here are a few links you might want to scan:
Except for one of those links, all say you need to have an app that can actually access the GPS info in your phone. Newer phones may be different. I suppose it's similar to the built-in accelerometer, there is no direct access to it in the installed apps.
At least one link suggests making your route while you have WiFi access. Then, if the app supports it, you don't even need a cellular connection to use the route (absolutely no data usage!).
Good luck! And, don't get lost! Many truckers have!